What are we doing?

The research team had to figure out how to measure the ozone absorbed from the air into the ocean. Until recently, direct measurements from a ship were impossible—the amounts of ozone exchanged were too tiny to measure, especially on a constantly moving ship. Scientists had to settle for taking measurements from coastal regions or trying to simulate conditions in a lab.  The team had to invent an instrument by borrowing some techniques from other industrial and scientific instruments.  After a lot of work, they came up with an ozone analyzer that is fast and sensitive enough to measure ozone fluxes in mid-ocean from a research ship.

The instruments

The ozone analyzer works by measuring a reaction between the ozone in the air just above the surface of the water and nitric oxide (NO) contained in the instrument.  When a molecule of NO meets a molecule of ozone, they react to produce molecules of oxygen and nitrogen dioxide and one photon of light. The researchers just have to count the number of photons produced to see how many molecules of ozone were present when the measurement was taken. A single photon is too tiny to see, however, so they convert it into an electron, which is then multiplied by 2 to the 12th power (about 4,100 times). That produces an electric current in waves that are strong enough to measure. On the instrument display, the waves look a bit like a fuzzy train with several cars. Each “train car” equals one molecule of ozone.

The researchers know that local conditions at each place on the sea affect ozone fluxes: the composition of the water, stirring of the water by currents and winds, and the amount of plankton present all have an impact. So, as well as measuring the ozone moving between air and water, the team measures all kinds of local variables. They mount the ozone analyzer in a bundle of instruments that measure many things at the same time—wind speed, air temperature, humidity, and even the motion of the ship. The team also samples the water to see the amount of plankton in it.

The team of scientists is taking the instrument bundle on several sea voyages all around the world.  They hope to see how the different conditions at different latitudes change the amount of ozone moving between air and sea.

Finding a proxy

The scientists suspect that the amount of plankton may be especially important to how much ozone the ocean absorbs. They think that when plankton dies, the decomposition process changes ozone into other chemicals. So the more plankton there are, the more ozone gets absorbed.

Plankton are tiny—it usually takes a microscope to see them. But they are present in such massive numbers that together they form most of the life in the ocean. Plankton exist throughout the world’s oceans, but they are concentrated in colder regions near the poles. If the tiny plankton have a big effect on ozone absorption, the scientists believe, then more ozone is absorbed near the poles than near the equator.

The research team hopes that plankton will be the key to measuring ozone fluxes in the future. Ocean voyages cost too much to repeat more than a few times - just enough to prove that the concepts the scientists are using work. The researchers want to tie their results to something easy and less expensive to measure, such as remote sensing data from orbiting satellites. If they can do that, it will be easier to incorporate their work into global climate models.

The team hopes that chlorophyll maps might serve as a proxy for ozone fluxes. In remote sensing images, we can see where plankton are concentrated by measuring chlorophyll, which many kinds of plankton use to convert carbon dioxide and water into food energy.

If ozone flux measurements correspond with the presence of plankton in the water, and plankton can be detected by measuring chlorophyll, then the scientists can rely on chlorophyll maps to tell them where ozone fluxes are concentrated.

The instrument packet the team is using to measure ozone fluxes includes, from top to bottom, an anemometer/thermometer that measures wind speed and air temperature, a motion sensor in a cylindrical canister that tracks the movement of the ship, a mean air temperature and humidity probe, a fast-response sensor of carbon dioxide and water, and a fast-response infrared hygrometer to record the amount of water vapor in the air. Photo by Chris Fairall.

The research vessel Ronald H. Brown. Photo by Chris Fairall.

Ozone concentrations for three days in January over the Pacific Ocean as depicted in a climate model developed by Laurens Ganzveld and J. Lelieveld. The y-axis is elevation above the sea surface up to 1 km. Time runs to the right along the x-axis. Darker red colors show lower ozone values. The model illustrates how ozone absorption relies on organic material in the water (namely, plankton) rather than being simply distributed across the surface of the water.

This chlorophyll map shows the concentration of chlorophyll in phytoplankton (plankton that are tiny plants). The image was taken using the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) on a satellite. The blue areas near the equator show low chlorophyll levels; green and yellow areas indicate increased levels; and red indicates high levels. Image courtesy of the SeaWiFS Project, NASA/Goddard Space Flight Center